Deliquescence, defined

A deliquescent material takes up moisture freely in an atmosphere with a relative humidity above a specific, well-defined critical point. That point for a given substance is defined as the critical relative humidity (RH0). Relative humidity (RH) is defined as the ratio of water vapor pressure in the atmosphere divided by water vapor pressure over pure water times 100% [RH = (PJP0) X 100%]. Once moisture is taken up by the material, a concentrated aqueous solution of the deliquescent solute is formed. The mathematical models used to describe the rate of moisture uptake involve both heat and mass transport. 

Arsenic pentoxide is a white deliquescent solid. It exhibits no basic properties, but forms arsenates analogous to the ortho-, meta-and pyro-phosphates. The formation of a pentachloride has not been established, but the pentafluoride has been obtained and is well defined. 

In terms of salt selection procedure, the critical relative humidity (CRH) of each salt should be identified. This is defined as the point at which the compound starts to sorb moisture. Clearly, compounds or salts that exhibit excessive moisture uptake should be rejected. The level of this uptake is debatable, but those exhibiting deliquescence (where the sample dissolves in the moisture that has been sorbed) should be automatically excluded from further consideration. 

The phosphonium iodides form addition products with the iodides of mercury, bismuth and thallium, and also vdth bismuth chloride. Perhalides, R PI.Xg, are formed when halogens react with the iodides, these products, like the original iodides, having well-defined crystalline forms moist silver oxide removes the iodine, gnung the corresponding hydroxides. Like the arsonium chlorides the tetra-alkylphosphonium chlorides tend to be deliquescent, but they yield crystalline double salts vdth the chlorides of gold, mercury, platinum, bismuth and zinc. When two molecular equivalents of triethylphosphine react with one molecular equivalent of methylene chloride, ethylene dichloride or ethylene dibromide, phosphonium compounds result in which two atoms of phosphorus are present. 

Atmospheric corrosion rates are commonly related to a critical relative humidity , above which the corrosion rate increases significantly and below which the rate is insignificant for many practical purposes. Depending on metal and exposure conditions, critical relative humidities have been reported in the range from 50 to 90%. The critical relative humidity is associated with the point of deliquescence of deposited aerosol particles, above which the aerosols rapidly absorb water until a saturated solution is obtained. For a single-phase aerosol, there is a well-defined critical relative humidity, whereas for a mixture of phases (the common situation in natural outdoor environments) the critical relative humidity is lower than those of the single phases. 

There is an important caveat in all the dehquescence studies of NaCl reviewed by Martin. The samples were polycrystalhne, or if a single grain, their surfaces were irregular and not defined. Since it is known that surface curvature, steps and defects can effect interfacial properties [2,3,6,26,27,44], Cantrell et al. [89] undertook to study dehquescence of cleaved NaCl (001) faces placed in a temperature and humidity controlled environment. The onset of dissolution (deliquescence) was monitored by the appearance of a thick film of brine as determined from its infrared spectrum. The infrared signatures of neat water and brine are quite distinct, so the film grown was brine and not water. Photometry was used to determine the brine thickness which... 

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